1. Field of the Invention
The present invention relates to a method of making a cylindrical ferromagnetic body and cover assembly that is used for making a permanent-magnet rotor of a dc brushless motor.
2. Description of the Prior Art
A conventional permanent-magnet rotor of a dc brushless motor has a cylindrical iron core for providing magnetic flux paths and a cylindrical permanent magnet, or a plurality of arcuate permanent magnets, coaxially positioned around the iron core and bonded thereto.
The permanent magnet is externally covered with a non-magnetic cylindrical protective sleeve or cover so that the permanent magnet may be prevented from being damaged while the rotor is assembled to the stator or any magnet fragments may be kept from scattering when the permanent magnet is accidentally broken while it is in rotation. The use of a cylindrical protective cover is particularly necessary in the case a magnet containing a plastic binder is used because such a magnet is more likely to be accidentally damaged. In order to obtain a good dynamic balancing of the rotor, the magnet and the cylindrical cover must be dimensionally very precisely and physically very tightly assembled with each other.
One of the conventional methods is disclosed by Stark in U.S. Pat. Nos. 5,040,286 and 5,144,735, in which a cylindrical retaining shell is axially cold-pressed over an assembly of cylindrically arranged arcuate magnetizable elements. The inside diameter of the retaining shell is slightly smaller than the outside diameter of the assembly of the magnetizable elements so that the retaining shell is radially outwardly stretched, thereby tightly retaining the magnetizable elements.
Another conventional method is to cover the cylindrical magnet or magnetizable material with a cylindrical sleeve by a heat-shrinking method.
The problems to be solved
However, the cold-pressing method requires a very strict machining tolerance of the cylindrical cover and the heat-shrinking method requires a strict temperature control so that the cylindrical cover may not be damaged or overly strained.
Furthermore, both of the above mentioned cold-pressing method and the heat-shrinking method require extra process steps in addition to the steps for forming or making the magnetizable material or the magnet.